The present invention relates to air bearing sliders, and more particularly to air bearing sliders used in production glide testing of discs for disc drives.
Many systems use structures which are required to have a substantially flat surface. In particular, many disc drives use air bearing sliders to support a transducer, such as a magnetic read/write head, over the media disc surface. The air bearing slider rides on an extremely thin layer of air or "wind" which "sticks" to and travels with the rotating disc. For proper operation of the air bearing slider and read/write head, the disc surface over which the air bearing slider travels must be substantially flat.
With improvements to disc fabrication techniques, increasingly flat surfaces are possible. Nonetheless, current economical disc fabrication techniques do not provide discs that are sufficiently flat in all instances. Microscopic bumps or asperities remain on the disc surface after fabrication. If an asperity is large enough, it can cause read/write head to disc contact during operation of the disc drive. The read/write head to disc contact results at a minimum in degraded disc drive performance such as data loss and, if sufficiently severe, results in a fatal disc drive failure or "crash". Due to various reasons, even asperities which are smaller than the nominal read/write head fly height may still cause similar problems degrading disc drive performance.
The read accuracy and recorded density of a read/write head generally improves with lower fly height. It is accordingly continually desired to position read/write heads closer to disc or media surfaces. Fly heights as low as 0.7 micro inches (180 .ANG.) may be possible. As the distance between the slider and the disc becomes smaller, disc surface tribology becomes more critical to slider aerodynamics and performance.
In typical disc drives, the disc is rotated during use at a uniform rotation speed or angular velocity, regardless of the radial position of the slider. The slider is moved between different radial tracks to read and write data to different locations on the disc. For maximum accuracy of recording, it is desired that the air bearing slider maintain a selected uniform flying height at all radii of the disc. Different radii of the disc correspond to different wind velocities or linear disc speeds in circumferential inches per second. Most air bearing sliders for use in disc drives are intended to be relatively insensitive to variations in wind speed, so as to fly at the same flying height at an outer radius as at an inner radius.
Much effort has gone into the design of air bearing sliders to improve flight characteristics. For instance, many slider designs include an inside rail and an outside rail separated by a central cavity. One slider design, known as a "catamaran" slider, includes two rails of constant width separated by a cavity of near ambient pressure. The rails generate a positive pressure lift force for the slider. One of the rails may ride lower than the other or have the active transducer positioned thereon, and thus be an "active" rail of the slider. Some air bearing sliders include a cross rail or crossbar which forms a large air expansion region of generally negative pressure. The magnitude of the negative pressure increases at the greater wind velocities present at outer radii, offsetting an increase in the positive pressure lift force of the rails, and thus the crossbar aids in making the slider relatively insensitive to wind speed.
Additionally, prior disc drives can be categorized according to the type of actuation--rotary actuated and linearly actuated. Rotary actuated sliders are carried on an actuator arm which pivots about a fixed actuator arm pivot point, causing the slider to access across the surface of the disc in an arc. With a rotary actuated head, a "skew" angle (i.e., the angle between the longitudinal axis of the slider and a tangent to the track) changes as a function of the slider's radial position on the disc. Much design work has been directed at maintaining constant flying performance despite changing skew angles. Linearly actuated heads access radially inward and outward on the disc in a line rather than in an arc. With a linearly actuated slider, the skew angle is maintained constant at all radii, such as 0.degree..
To ensure that only discs having a sufficiently flat surfaces are used in production disc drives, discs are production tested in a quality control procedure prior to installation in a disc drive. The smoothness testing is performed with a "glide test", which involves flying an air bearing "glide" slider over the disc. The glide slider includes a piezo-electric element or other vibration sensor, which may be bonded on the back of the air bearing slider. The vibration sensor detects head-disc interferences or hits. Each instance of vibration is sensed and recorded for either remedial action or toward rejection of the disc. Vibration may be caused by bumps which decrease the clearance of the slider. If a bump is sufficiently large to create eddy currents and swirls in the air stream which cause the glide slider to vibrate, then the bump is recorded as a defect. The glide slider typically flies at a height lower than the read/write head during normal conditions in order to ensure that any asperity high enough to contact the read/write head will be detected.
Prior to the glide test, a cycle of burnish is performed to remove larger defects or hard particles. If defects are found during the glide test, another burnish process may be performed to remove the asperity. The glide test may then be performed again, to ensure that the asperity was removed. If the burnish process is unsuccessful, the disc may be rejected as not meeting the quality standard required for the production disc drive. Production glide/burnish testing of discs is a time consuming and expensive process, but is necessary for high quality control standards of disc drive products.
Burnish/glide testers are commercially provided such as MC900-T3 by Phase Metrics Corporation of Fremont, Calif. This glide testing is performed with a catamaran type air bearing slider, having rail widths of 11 to 12 mils and flying heights of 1.1-1.7 micro inches with a type 2 suspension 9.5 gram load and a speed from 400 ips to 600 ips. In the burnish/glide tester, the rotational speed of the disc is changed inversely proportional to the radius being accessed by the slider, so that the slider sees a uniform wind velocity despite changing radial locations along the disc.
The glide testing track width, i.e., the incremental difference in radial location of the glide head from rotation to rotation, is about 2 to 3 mils. With a 12 mil active rail width and a 3 mil glide testing track width, the active rail will pass over each defect a minimum of a four times.